96 On the Land and Fresh-water Mollusca
the district, and no species of Vertigo has been recorded.
Some species exist in all probability, but none of my friends
who have searched for shells have found them, nor have I
been more fortunate, although in other parts of the country
I have experienced no difficulty in detecting Vertigos when
inhabiting the locality.
Balia perversa, Linn.—I found this snail once, commonly,
under the bark of a dead poplar, near the Decoy, West
Mersea, and this is the only situation in which I know of its
occurrence.
Clausilia rugosa, Drap.—Locally abundant, and occur-
ring in many parts of my district sparingly. I think they are
most frequent on the Boulder Clay formation. I have not
recognized any of the named varieties.
Cochlicopa tridens, Pult.—I have no doubt this occurs,
although I have never found it. It occurs sparingly at
Wiston, on the Suffolk side of the Stour.
C. lubrica, Mull.—Frequent in all parts of the district, but
I have not been able to distinguish any of the named
varieties.
Achatina acicula, Mull.—I have never met with the living
snail, but dead shells are found in all parts of the district.
It is said to occur only on calcareous soils, but I find the
shells most frequently in the sandy and gravelly soil around
the town. In the rubbish and growing sods of grass on the
Roman wall it is frequently noticed, and some years since a
Roman cloaca near the castle was opened, and in the soil at
the bottom of the drain this shell occurred in thousands.
There is one way to find it that I have never seen mentioned.
It is to search the tops of the hills of the small ants in the
open fields, on any soil, and amongst the earth the insects
bring up the shells may generally be found, frequently
washed clean by the rain which has percolated the soil.
Carychiidae.
Carychium minimum, Mull.—Common everywhere in damp
spots in meadows, hedges and woods. Easily found, by
employing the winnowing process.